SwRI scientist: Pluto-Charon origin may
mirror that of Earth and its Moon

Boulder, Colo. --
January 27, 2005 - The evolution of Kuiper Belt objects, Pluto and its lone moon
Charon may have something in common with Earth and our single Moon: a giant
impact in the distant past.

Dr. Robin Canup,
assistant director of Southwest Research Institute's® (SwRI®)
Department of Space
Studies, argues for such an origin for the Pluto-Charon pair in an article for
the January 28 issue of the journal Science.

Canup, who currently
is a visiting professor at the California Institute of Technology, has worked
extensively on a similar "giant collision" scenario to explain the Moon's
origin.

In both the Earth-Moon
and Pluto-Charon cases, Canup's smooth particle hydrodynamic simulations depict
an origin in which a large, oblique collision with the growing planet produced
its satellite and provided the current planet-moon system with its angular
momentum.

While the Moon has
only about 1 percent of the mass of Earth, Charon accounts for a much larger 10
to 15 percent of Pluto's total mass. Canup's simulations suggest that a
proportionally much larger impactor - one nearly as large as Pluto itself - was
responsible for Charon, and that the satellite likely formed intact as a direct
result of the collision.

According to Canup, a
collision in the early Kuiper Belt - a disk of comet-like objects orbiting in
the outer solar system beyond Neptune - could have given rise to a planet and
satellite with relative sizes and angular rotation characteristics consistent
with those of the Pluto-Charon pair. The colliding objects would have been about
1,600 to 2,000 kilometers in diameter, or each about half the size of the
Earth's Moon.

"This work suggests
that despite their many differences, our Earth and the tiny, distant Pluto may
share a key element in their formation histories. This provides further support
for the emerging view that stochastic impact events may have played an important
role in shaping final planetary properties in the early solar system," said
Canup.

The "giant impact"
theory was first proposed in the mid-1970s to explain how the Moon formed, and a
similar mode of origin was suggested for Pluto and Charon in the early 1980s.
Canup's simulations are the first to successfully model such an event for the
Pluto-Charon pair.

Simulations published
by Canup and a colleague in Nature in 2001 showed that a single impact by a
Mars-sized object in the late stages of Earth's formation could account for the
iron-depleted Moon and the masses and angular momentum of the Earth-Moon system.

This was the first
model to simultaneously explain these characteristics without requiring that the
Earth-Moon system be substantially modified after the lunar forming impact.

This research was
supported by the National Science Foundation under grant no. AST0307933.